A most preferred example of the above communications system discussed in the present invention is a mobile communications system. The following explanation will be made by taking as an example this mobile communications system. Accordingly, if according to this example, the communications apparatus is a (i) base station (or a higher base station controller thereof) and/or a (ii) mobile station (including a mobile terminal such as a PDA). Note that for convenience, in the later explanation, the former (i) will be referred to as the “base station” and the latter (ii) will be simply referred to as the “terminal” in some cases. Note that, as will be clear in the later explanation, the present invention can be substantially equivalently applied to not only the above base station, but also the above terminal. It is not particularly necessary to differentiate between the two.
In a mobile communications system, securing a desired transmission rate for a user is a major issue in providing it with service. On the other hand, usually the used frequency band used by the mobile communications system is fixed for each system. Therefore, even if employing user multiplexing etc., the maximum transmission rate thereof ends up being restricted. For this reason, the method of flexibly changing the used frequency band in accordance with the required transmission rate is being studied.
Further, when considered by the mobile communications system as a whole, the state of usage differs for each used frequency band. Sometimes a band is not used at all. For this reason, from the viewpoint of the effective utilization of the frequency, it has been studied to make the used frequency band variable.
Under this situation, technology of making the used frequency band variable in an MC (Multi-Carrier)-CDMA (Code Division Multiple Access) or OFDM (Orthogonal Frequency Division Multiplex) or other multicarrier transmission mobile communications system has been proposed. For example, the methods disclosed in the following four Patent Documents 1 to 4 are proposed. Details thereof will be explained later with reference to the drawings, but these may be summarized as follows:
1) A “multiple connection method and apparatus” disclosed in Patent Document 1 is characterized by dividing a series of subcarriers so as to freely assign used frequency bands to users.
2) A “mobile station, base station, and mobile communications network” disclosed in Patent Document 2 are characterized in that a subcarrier band dedicated to transmission of control signals is set in the communications network.
3) A “channel allocation method” disclosed in Patent Document 3 is characterized by changing the number of subcarriers in the series of subcarriers in accordance with length of the communications distance between the base station and the mobile station.
4) A “wireless transmission apparatus and wireless communications method” disclosed in Patent Document 4 is characterized by changing the bandwidth of each subcarrier in the series of subcarriers to make the bandwidth of the used frequency band variable.    [Patent Document 1] Japanese Patent Publication (A) No. 9-205411    [Patent Document 2] Japanese Patent Publication (A) No. 2003-264524    [Patent Document 3] Japanese Patent Publication (A) No. 2004-21476    [Patent Document 4] Japanese Patent Publication (A) No. 2002-330467